What is Crowding, crowded trade, comomentum, unwind?

Crowding is the presence of many investors in the same positions; a crowded trade is a position held by enough of them that their joint exit would move prices. Comomentum (Lou and Polk) is the average correlation of the abnormal returns of the stocks a momentum strategy holds, a measure of how many traders act on them. An unwind is the rapid reduction of a crowded position by some of its holders, which moves the prices at which the others mark theirs.